TrackedOp: allow dumping historic ops sorted by duration

TrackedOp: allow dumping historic ops sorted by duration

Currently dump_historic_ops dumps ops sorted by their initiation time,
which may not have any relation to how long it took, and sorting output
of that command by op duration is neither fast nor convenient.
New asok command ("dump_historic_ops_by_duration") outputs the same
op list, but ordered by their duration time (longest first).

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>